The major objectives of this investigation are threefold, and are aimed at testing the hypotheses that the activity of brainstemthermosensitive neurons controls the level of body temperature, and that changes in body temperature produced by intracerebral microinjected amines are due to an action on these neurons. The first objective is to observe the spontaneous spike activity, and to test the thermal sensitivity, of individual brainstem neurons of unanesthetized, euthermic (non-hibernating) ground squirrels (Citellus lateralis) and of unanesthetized ground squirrels during hibernation and arousal from hibernation. Extracellular single unit recordings will be made in the preoptic and anterior hypothalamus (PO/AH) and the mesencephalic reticular formation (MRF) with multibarrel micropipettes before, during , and after local heating and cooling of these structures by indwelling thermodes. The responsiveness of the cells to local thermal changes will be categorized according to the physical characteristics of the curve (positive or negative slope, continuous or discontinuous function) relating firing rate to locat temperature. The second objective is to test the responsiveness of individual brainstem neurons to iontophoretically applied biogenic amines in unanesthetized, euthermic ground squirrels and in unanesthetized ground squirrels in hibernation and during arousal from hibernation. Emphasis will be placed on testing those amines which considered to be likely candidates for brainstem neural transmitters (acetylcholine, norepinephrine, and 5-hydroxytryptamine). The third objective is to study, in unanesthetized euthermic and hibernating ground squirrels, the thermoregulatory effects of biogenic amines microinjected through chronically implanted cannulae into PO/AH and MRF sites known to contain amine-sensitive, thermosensitive neurons.